Surface modification of Li[Ni0.3Co0.4Mn0.3]O2 cathode by Li-La-Ti-O coating

A Li[Ni_0.3Co_0.4Mn_0.3]O₂ cathode is modified by applying a Li-La-Ti-O coating using the hydrothermal method. The coated Li[Ni_0.3Co_0.4Mn_0.3]O₂ is characterized by X-ray diffraction analysis, scanning electron microscopy, energy-dispersive spectrometry, transmission electron microscopy, and differential scanning calorimetry. The Li-La-Ti-O coating layer is formed as crystalline (perovskite structure) or amorphous phase depending on the heating temperature. The Li-La-Ti-O coated Li[Ni_0.3Co_0.4Mn_0.3]O₂ electrode has better rate capability than the pristine electrode. In particular, the rate capability is significantly associated with heating temperature; this is probably due to the phase of the coating layer. It appears that the Li-La-Ti-O coating of amorphous phase is superior to that of crystalline phase for obtaining enhanced rate capability of the coated samples. The thermal stability and cyclic performance of the Li[Ni_0.3Co_0.4Mn_0.3]O₂ electrode are also improved by Li-La-Ti-O coating. These improvements indicate that the Li-La-Ti-O coating is effective in suppressing the chemical and structural instabilities of Li[Ni_0.3Co_0.4Mn_0.3]O₂.     

The electrochemical property of ZrFx-coated Li[Ni1/3Co1/3Mn1/3]O2 cathode material

Electrochemical and thermal properties of pristine and ZrF_x-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials are compared. The hydrothermal method is introduced for the fabrication of a uniform coating layer. The formation of a compact coating layer on the surface of pristine powder is observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From TEM-EDS and XPS analysis, it is inferred that the coating layer is ZrO_xF_y (zirconium oxyfluoride) form. The coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrodes have better rate capability and cyclic performance at high temperature compared with the pristine electrode. The thermal stability of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode is also enhanced by the ZrF_x coating. Such enhancements are due to the presence of a stable coating layer, which effectively suppresses the chemical instability ascribed to surface reaction between electrode and electrolyte.     

Enhanced electrochemical properties of Li(Ni0.4Co0.3Mn0.3)O2 cathode by surface modification using Li3PO4-based materials

The surface of a commercial Li[Ni_0.4Co_0.3Mn_0.3]O₂ cathode is modified using Li₃PO₄-based coating materials. The electrochemical properties of the coated materials are investigated as a function of the pH value of the coating solution and the composition of coating materials. The Li₃PO₄ coating solution with pH 2 is found to be favorable for the formation of stable coating layers having enhanced electrochemical properties. The Li₃PO₄, Li_1.5PO₄, and PO₄ coating layers are formed as amorphous phases. However, the Li_3−xNi_x/2PO₄ coating layers are composed of small particles with a crystalline phase covered with an amorphous phase. Li₃PO₄ and Li_1.5PO₄ coatings considerably enhance the rate capability of the Li[Ni_0.4Co_0.3Mn_0.3]O₂ electrode. In contrast, the Li_3−xNi_x/2PO₄ coating material, which contained Ni, has an inferior rate capability compared to the Li_xPO₄ series (x = 1.5 and 3), although the LiNiPO₄-coated electrode shows a better rate capability than a pristine one. Li₃PO₄-based coating materials are effective at enhancing the cyclic performance of the electrode in the voltage range of 3.0-4.8 V. DSC analysis also confirms the improved thermal stability attained by coating the cathode with Li₃PO₄-based materials.     

Structural and thermal properties of LiNi0.6−xMgxCo0.25Mn0.15O2 cathode materials

For improving the electrochemical performance and thermal stability, magnesium was chosen as the doping element in Li(NiCoMn)O₂ cathode materials. LiNi_0.6−xMg_xCo_0.25Mn_0.15O₂ (x = 0 and 0.03) were successfully synthesized via the mixing hydroxide method. These materials exhibited α-NaFeO₂ structure as indicated by the XRD patterns. The intensity ratio of (0 0 3) to (1 0 4) showed that the Mg substitution could reduce the cation mixing. The pristine material exhibited the initial discharge of capacity 199 mAh g⁻¹ and remained retention of 79% after 20 cycles in the voltage range of 3–4.5 V. When magnesium ions were substituted, the initial capacity was reduced due to the less active ions. However, the capacity retention was increased to 95%. Not only cycleability, but also the thermal stability was improved by Mg substitution at every delithiated state of electrodes with electrolytes. The in situ synchrotron X-ray diffraction patterns showed that the boundary of phase transition for H1 to H2 was much clearer in Mg-doped sample, indicating that the LiNi_0.57Mg_0.03Co_0.25Mn_0.15O₂ material exhibited higher structural integrity. The improvements of both electrochemical retention and thermal stability were possibly attributed to the reduced cation mixing and complete structural changes.     

Influence of synthesis conditions on electrochemical properties of high-voltage Li1.02Ni0.5Mn1.5O4 spinel cathode material

Li_1.02Ni_0.5Mn_1.5O₄ spinel cathode materials were successfully synthesized by a citric acid-assisted sol-gel method. The structure and morphology of the materials have been examined by X-ray diffraction and scanning electron microscopy, respectively. Electrochemical properties of the materials were investigated using cyclic voltammetry and galvanostatic charge/discharge measurements at two different temperatures (25 and 55 °C) using lithium anode. The initial capacity and capacity retention are highly dependent on the particle size, particle size distribution, crystallinity and purity of the materials. The Li_1.02Ni_0.5Mn_1.5O₄ materials synthesized both at 800 and 850 °C have shown best electrochemical performance in terms of capacity and capacity retention between 3.5 and 4.9 V with a LiPF₆ based electrolyte.     

Synthesis and electrochemical performances of core-shell structured Li[(Ni1/3Co1/3Mn1/3)0.8(Ni1/2Mn1/2)0.2]O2 cathode material for lithium ion batteries

Micro-scale core-shell structured Li[(Ni_1/3Co_1/3Mn_1/3)_0.8(Ni_1/2Mn_1/2)_0.2]O₂ powders for use as cathode material are synthesized by a co-precipitation method. To protect the core material Li[Ni_1/3Co_1/3Mn_1/3]O₂ from structural instability at high voltage, a Li[Ni_1/2Mn_1/2]O₂ shell, which provides structural and thermal stability, is used to encapsulate the core. A mixture of the prepared core-shell precursor and lithium hydroxide is calcined at 770 °C for 12 h in air. X-ray diffraction studies reveal that the prepared material has a typical layered structure with an space group. Spherical morphologies with mono-dispersed powders are observed in the cross-sectional images obtained by scanning electron microscopy. The core-shell Li[(Ni_1/3Co_1/3Mn_1/3)_0.8(Ni_1/2Mn_1/2)_0.2]O₂ electrode has an excellent capacity retention at 30 °C, maintaining 99% of its initial discharge capacity after 100 cycles in the voltage range of 3-4.5 V. Furthermore, the thermal stability of the core-shell material in the highly delithiated state is improved compared to that of the core material. The resulting exothermic onset temperature appear at approximately 272  °C, which is higher than that of the highly delithiated Li[Ni_1/3Co_1/3Mn_1/3]O₂ (261 °C).     

Cyclic deterioration and its improvement for Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2

Transmission electron microscopy (TEM) studies were carried out to elucidate cyclic deterioration phenomena for Li-rich layered cathode material Li[Ni_0.17Li_0.2Co_0.07Mn_0.56]O₂. The results obtained show that the deterioration starts during the initial charge/discharge to higher potential over 4.5 V, and leads to the formation of micro-cracks at the crystal surface and the distortion of crystal periodicity. These two kinds of defects lead to further non-crystallization of the crystal surface and the emergence of a very small amount of another possible phase. Our stepwise pre-cycling treatment effectively depressed the formation of the former two kinds of defects, and could significantly improve cyclic durability. The observation of non-crystallization at the cathode crystal surface, which would diminish the battery performance, is consistent with our preliminary ac impedance results.     

Synthesis and electrochemical properties of Li[Ni0.45Co0.1Mn0.45−xZrx]O2 (x = 0, 0.02) via co-precipitation method

Li[Ni_0.45Co_0.1Mn_0.45−xZr_x]O₂ (x = 0, 0.02) was synthesized via co-precipitation method. Partial Zr doping on the host structure of Li[Ni_0.45Co_0.1Mn_0.45]O₂ was carried out to improve the electrochemical properties. The Zr-doped Li[Ni_0.45Co_0.1Mn_0.43Zr_0.02]O₂ was evaluated in terms of specific discharge capacity, cycling performance and thermal stability. The Zr-doped Li[Ni_0.45Co_0.1Mn_0.45−xZr_0.02]O₂ shows the improved cycling performance and stable thermal stability. The major exothermic reaction was delayed from 252.1 °C to 289.4 °C.     

In situ X-ray absorption spectroscopic study of Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2

Although Li-rich solid-solution layered materials Li₂MnO₃-LiMO₂ (M = Co, Ni, etc.) are expected as large capacity lithium insertion cathodes, the fundamental charge-discharge reaction mechanism of these materials is not clear. Therefore the change in valence states of Ni, Co and Mn of Li[Ni_0.17Li_0.2Co_0.07Mn_0.56]O₂ during charge-discharge was examined in detail using in situ X-ray absorption spectroscopy (XAS), which includes both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Since the Mn K edge shift during charge-discharge was not clear to determine the valence change of Mn, the Mn K pre-edge shift was examined during charge-discharge. In our measurements, only a small shift of the Mn K pre-edge toward lower energy was observed on discharge from 4.8 to 2.0 V. This corresponds to a decrease of the Mn valence from 4+ to approximately 3.6+. However, this shift cannot explain the large reversible capacity of this material and thus strongly suggests the participation of oxygen in the reversible charge-discharge reaction of this material.     

Morphology and electrochemical performance of Li[Ni1/3Co1/3Mn1/3]O2 cathode material by a slurry spray drying method

The spherical Li[Ni_1/3Co_1/3Mn_1/3]O₂ powders with appropriate porosity, small particle size and good particle size distribution were successfully prepared by a slurry spray drying method. The Li[Ni_1/3Co_1/3Mn_1/3]O₂ powders were characterized by XRD, SEM, ICP, BET, EIS and galvanostatic charge/discharge testing. The material calcined at 950 °C had the best electrochemical performance. Its initial discharge capacity was 188.9 mAh g⁻¹ at the discharge rate of 0.2 C (32 mA g⁻¹), and retained 91.4% of the capacity on going from 0.2 to 4 C rate. From the EIS result, it was found that the favorable electrochemical performance of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode material was primarily attributed to the particular morphology formed by the spray drying process which was favorable for the charge transfer during the deintercalation and intercalation cycling.     

Synthesis and electrochemical studies of the 4 V cathode, Li(Ni2/3Mn1/3)O2

Layered Li(Ni_2/3Mn_1/3)O₂ compounds are prepared by freeze-drying, mixed carbonate and molten salt methods at high temperature. The phases are characterized by X-ray diffraction, Rietveld refinement, and other methods. Electrochemical properties are studied versus Li-metal by charge–discharge cycling and cyclic voltammetry (CV). The compound prepared by the carbonate route shows a stable capacity of 145 (±3) mAh g⁻¹ up to 100 cycles in the range 2.5–4.3 V at 22 mA g⁻¹. In the range 2.5–4.4 V at 22 mA g⁻¹, the compound prepared by molten salt method has a stable capacity of 135 (±3) mAh g⁻¹ up to 50 cycles and retains 96% of this value after 100 cycles. Capacity-fading is observed in all the compounds when cycled in the range 2.5–4.5 V. All the compounds display a clear redox process at 3.65–4.0 V that corresponds to the Ni^2+/3+–Ni^3+/4+ couple.     

Improvement of structural and electrochemical properties of AlF3-coated Li[Ni1/3Co1/3Mn1/3]O2 cathode materials on high voltage region

The particle surface of Li[Ni_1/3Co_1/3Mn_1/3]O₂ was modified by AlF₃ as a new coating material to improve the electrochemical properties in the high cutoff voltage of 4.5 V. The AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ showed no difference in the bulk structure compared with the pristine one and the uniform AlF₃ coating layers whose thickness is of about 10 nm covered Li[Ni_1/3Co_1/3Mn_1/3]O₂ particles, as confirmed by a transmission electron microscopy. The AlF₃ coating on Li[Ni_1/3Co_1/3Mn_1/3]O₂ particles improved the overall electrochemical properties such as the cyclability, rate capability and thermal stability compared with those of the pristine Li[Ni_1/3Co_1/3Mn_1/3]O₂. Such enhancements were attributed to the presence of the stable AlF₃ layer which acts as the interfacial stabilizer on the surface of Li[Ni_1/3Co_1/3Mn_1/3]O₂.     

Electrochemical and thermal characterization of AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cathode in lithium-ion cells

Li[Ni_0.8Co_0.15Al_0.05]O₂ particles are modified with AlF₃ as a new coating material. Even though the initial discharge capacity of the coated Li[Ni_0.8Co_0.15Al_0.05]O₂ is almost the same as that of the pristine material, the capacity retention and the thermal stability, in a highly oxidized state are both significantly improved. This effect is attributed to the thin AlF₃ coating layer protecting the oxidized cathode particles from attack by hydrogen fluoride in the electrolyte.     

Synthesis and electrochemical performance of the high voltage cathode material Li[Li0.2Mn0.56Ni0.16Co0.08]O2 with improved rate capability

The high voltage layered Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode material, which is a solid solution between Li₂MnO₃ and LiMn_0.4Ni_0.4Co_0.2O₂, has been synthesized by co-precipitation method followed by high temperature annealing at 900 °C. XRD and SEM characterizations proved that the as prepared powder is constituted of small and homogenous particles (100-300 nm), which are seen to enhance the material rate capability. After the initial decay, no obvious capacity fading was observed when cycling the material at different rates. Steady-state reversible capacities of 220 mAh g⁻¹ at 0.2C, 190 mAh g⁻¹ at 1C, 155 mAh g⁻¹ at 5C and 110 mAh g⁻¹ at 20C were achieved in long-term cycle tests within the voltage cutoff limits of 2.5 and 4.8 V at 20 °C.     

Structural characteristics and electrochemical performance of layered Li[Mn0.5−xCr2xNi0.5−x]O2 cathode materials

Li[Mn_0.5−xCr_2xNi_0.5−x]O₂ (0 < 2x <0.2) (Mn/Ni = 1) cathode materials have been synthesized by a solution method. X-ray diffraction patterns of the as-prepared materials were fitted based on a hexagonal unit cell (α-NaFeO₂ layer structure). The extent of Li/Ni intermixing decreased, and layering of the structure increased, with increasing Cr content. Electrochemical cycling of the oxides, at 30 °C in the 3–4.3 V range vs. Li/Li⁺, showed that the first charge capacity increased with increasing Cr content. However, maximum discharge capacity (∼143 mAh g⁻¹) was observed for 2x = 0.05. X-ray absorption near edge spectroscopic (XANES) measurements on the K-edges of transition metals were carried out on pristine and delithiated oxides to elucidate the charge compensation mechanism during electrochemical charging. The XANES data revealed simultaneous oxidation of both Ni and Cr ions, whereas manganese remains as Mn⁴⁺ throughout, and does not participate in charge compensation during oxide delithiation.     

AlF3-coated LiCoO2 and Li[Ni1/3Co1/3Mn1/3]O2 blend composite cathode for lithium ion batteries

Surface modifications of electrode materials can improve the electrochemical and thermal properties of cathodes for use in lithium batteries. In this study, AlF₃-coated LiCoO₂ and AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials are blended, as both have the same crystal structure and exhibit similar electrochemical properties. The composite electrodes exhibit high discharge capacities of 180-188 mAh g⁻¹ in a voltage range of 3.0-4.5 V at room temperature. The capacity retention of the composite electrode is greater than 95% of the initial capacity after 50 cycles. The thermal stability of these composite electrodes is greatly improved because of the superior thermal stability of AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂. The blended AlF₃-coated LiCoO₂ and AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode shows two exothermic peaks, one at 227 °C from AlF₃-coated LiCoO₂ and another at 277 °C from AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂, accompanied by significantly reduced exothermic heat generation.     

Direct observation of the partial formation of a framework structure for Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2 upon the first charge and discharge

The structural changes upon the first charge and discharge of a Li-rich layered cathode material Li[Ni_0.17Li_0.2Co_0.07Mn_0.56]O₂ were investigated using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and selected-area electron diffraction (SAED). Atomic resolution STEM observations revealed that some of the transition metal (TM) atoms were transferred from the TM layers to the Li layers upon the first charge and discharge, leading to the partial formation of a framework structure. This framework structure was considered as a spinel structure based on our simulation results of the corresponding SAED pattern of the fully charged state. This framework structure was also recognized even at the early stage of the first charging process in the plateau region around 4.5 V by using the SAED patterns, indicating that the formation of this framework structure started at the same time as the electrochemical activation.     

Preparation and electrochemical properties of Li[Ni1/3Co1/3Mn1−x/3Zrx/3]O2 cathode materials for Li-ion batteries

Layer-structured Zr doped Li[Ni_1/3Co_1/3Mn_1−x/3Zr_x/3]O₂ (0 ≤ x ≤ 0.05) were synthesized via slurry spray drying method. The powders were characterized by XRD, SEM and galvanostatic charge/discharge tests. The products remained single-phase within the range of 0 ≤ x ≤ 0.03. The charge and discharge cycling of the cells showed that Zr doping enhanced cycle life compared to the bare one, while did not cause the reduction of the discharge capacity of Li[Ni_1/3Co_1/3Mn_1/3]O₂. The unchanged peak shape in the differential capacity versus voltage curve suggested that the Zr had the effect to stabilize the structure during cycling. More interestingly, the rate capability was greatly improved. The sample with x = 0.01 presented a capacity of 160.2 mAh g⁻¹ at current density of 640 mA g⁻¹(4 C), corresponding to 92.4% of its capacity at 32 mA g⁻¹(0.2 C). The favorable performance of the doped sample could be attributed to its increased lattice parameter.